Scissor lifting platforms are used in a variety of technical fields for lifting loads of different types and optionally persons. Different models of scissor lifting platforms are also used to lift motor vehicles, in particular motorcars, off-road vehicles and vans, in repair shops, production companies and also testing stations due to the simple lifting technology, the sturdy style and the possibility to arrange the retracted scissor lifting platform at floor level.
At least two congruent scissors are used for the construction of the lifting mechanism. If it is intended that particularly great heights be reachable, a plurality of such pairs of scissors can be arranged on top of each other which results e.g. in double scissor lifting platforms or multiple scissor lifting platforms.
In a lowered state, scissor lifting platforms should have an overall height as low as possible in order to facilitate the application of the loads to be lifted in this position. Especially the lifting platforms for motor vehicles are meant to protrude as little as possible beyond the surface of the floor in their lowered position in order to thus make it easier to drive on the motor vehicles. In this case, it is furthermore possible to omit a separate installation pit at the assembly site.
However, in the case of scissor lifting platforms the problem arises that the scissor bars which can be pivoted against to each other have to lie in parallel, if possible, in the lowered state, resulting in unfavorable lever geometries for the lifting units in the starting phase of the lifting movement.
Generally, the further the point of application of the lifting cylinder at the bearing site of the scissor arm is removed from the associated pivoting point, the more advantageous the leverage becomes and consequently the required forces for pushing out the lifting cylinder decrease.
In a known double scissor lifting platform bearings for an articulated connection of a lower scissor and an upper scissor of a base frame side are respectively positioned at the adjacent ends of the scissor bars formed as straight supports. Therefore, when the platform is lowered the scissor bars cannot move into a completely horizontal position because the bearings each rest on the top surfaces of the scissor bars of the lower scissor. Thus, the scissor bars remain in a slight inclination which determines the minimum height of the platform in the lowered position, that is, the overall height.
From this another problem results, namely that less favorable fulcrum conditions for the lifting cylinders result when the lifting height decreases so that much higher pressure forces are required for the lifting cylinder(s) for lifting the lifting table or the like pick-up of loads of a lifting platform from the lowered position thereof as compared to the nominal load. Therefore, conventional scissor lifting platforms cannot be retracted any more than down to a lowered position in which the lifting cylinder engaging with the lifting platform still has an angle of application of 5° or more.
DE 26 20 902 A1 shows a lifting device comprising a working platform the lifting mechanism of which is formed in a kind of double scissor. In the retracted position of the working platform, at the beginning of the movement auxiliary cylinders respectively disposed at the corners of the working platform lift the latter up to a particular height. As soon as a particular position has been reached the shearing force of the main cylinder acts on the bars of the frame and subsequently lifts them. The use of the additional auxiliary cylinders increases the manufacturing costs of the lifting device and prevents a compact structural style of the working platform.